The invention relates to a lithographic projection apparatus comprising a radiation source supplying a projection beam, a mask holder, a substrate holder and a projection system arranged between the mask holder and the substrate holder, the apparatus further comprising an alignment system for ultimately aligning a substrate, provided in the substrate holder, with respect to a mask provided in the mask holder, the alignment system comprising an off-axis alignment unit for aligning an alignment mark, provided on a substrate, with respect to a reference.
Ultimate alignment is understood to mean that, although in the first instance the alignment unit is used for aligning a substrate with respect to a reference, the result of this alignment step is used in combination with other measurements for aligning the substrate with respect to the mask.
The lithographic projection apparatus is an essential component in the manufacture of integrated circuits, or ICs, by means of diffusion and masking techniques. With the aid of this apparatus, a number of masks having different mask patterns are successively imaged at the same position on a semiconductor substrate. The substrate must undergo the desired physical and chemical changes between the successive images at the same position. To this end, the substrate must be removed from the apparatus after it has been exposed with a mask pattern, and, after it has undergone the desired process steps, the substrate must be replaced at the same position again so as to expose it with a second mask pattern, and so forth, while it must be ensured that the images of the second mask pattern and the subsequent mask patterns are positioned accurately with respect to the substrate. To this end, the lithographic projection apparatus is provided with an optical alignment system with which alignment marks on the substrate are aligned with respect to alignment marks on the mask.
A lithographic apparatus may not only be used for the manufacture of ICs but also for the manufacture of other structures having detailed dimensions of the order of 1 micrometer. Examples are structures of integrated, or plenary, optical systems or guiding and detection patterns of magnetic domain memories, and structures of liquid crystal display panels. Also in the manufacture of these structures, images of mask patterns must be aligned very accurately with respect to a substrate.
The lithographic projection apparatus may be a stepping apparatus or a step-and-scan apparatus. In a stepping apparatus, the mask pattern is imaged in one run on an IC area of the substrate. Subsequently, the substrate is moved with respect to the mask in such a way that a subsequent IC area will be situated under the mask pattern and the projection lens system and the mask pattern is imaged on the subsequent IC area. This process is repeated until all IC areas of the substrate are provided with a mask pattern image In a step-and-scan apparatus, the above-mentioned stepping procedure is also followed, but the mask pattern is not imaged in one run but via scanning movement. During imaging of the mask pattern, the substrate is moved synchronously with the mask with respect to the projection system and the projection beam, taking the magnification of the projection system into account. A series of juxtaposed partial images of consecutively exposed parts of the mask pattern is imaged in an IC area. After the mask pattern has been completely imaged in an IC area, a step is made to a subsequent IC area. A possible scanning procedure is described in the article: "Sub-micron 1:1 Optical Lithography" by D. A. Markle in the magazine "Semiconductors International" of May 1986, pp. 137-142.
U.S. Pat. No. 5,243,195 discloses an optical lithographic projection apparatus provided with an alignment system and intended for the manufacture of ICs. This alignment system comprises an off-axis alignment unit for aligning a substrate alignment mark with respect to this alignment unit. In addition, this alignment system comprises a second alignment unit for aligning a substrate mark with respect to a mask mark via the projection lens (TTL). Alignment via the projection lens (on-axis alignment) is the most frequently used method in the current generation of optical lithographic projection apparatuses and provides the advantage that the substrate and the mask can be aligned directly, and thus very accurately, with respect to each other. When the off-axis alignment method is used, the baseline offset as described in U.S. Pat. No. 5,243,195 must be taken into account.
The on-axis alignment method has hitherto worked to full satisfaction, but it is to be expected that this alignment method may present problems with regard to reliability and accuracy when novel technologies are used in the IC manufacture and when the detailed sizes, or line widths, of the IC patterns decrease.
In connection with the increasing number of electronic components per unit of surface area of the substrate and the resultant smaller dimensions of these components, increasingly stricter requirements are imposed on the accuracy with which integrated circuits are made. The positions where the successive masks are imaged on the substrate must therefore be fixed more and more accurately. In the manufacture of new-generation ICs with smaller line widths, the alignment accuracy will have to be improved or, in other words, it must be possible to detect smaller deviations so that the resolving power of the alignment system must be increased. On the other hand, stricter requirements must also be imposed on the planeness of the substrate due to the required higher numerical aperture (NA) of the projection lens system in the case of decreasing line widths. The depth of focus of this system decreases as the NA increases. Since some image field curvature occurs at the desired relatively large image field of the projection lens system, there is hardly any room left for unevennesses of the substrate. To obtain the desired planeness of the substrate, it has been proposed to polish this substrate by means of the chemical mechanical polishing (CMP) process between two consecutive exposures with different mask patterns in the projection apparatus. However, this polishing process affects the accuracy of the on-axis alignment method. In this method, a grating is used as a substrate alignment mark and the sub-beams diffracted in the first order by this grating are used for imaging the substrate mark on the mask mark. In this process, it is assumed that the substrate is aligned correctly with respect to the mask when the point of gravity of the substrate grating mark is aligned with respect to the point of gravity of the mask alignment mark. In that case it has been assumed that the point of gravity for each grating mark coincides with the geometric center of the grating. However, the CMP process renders the substrate grating mark asymmetrical so that this alignment method is no longer reliable.
Furthermore, the manufacturing process for new-generation ICs is becoming more and more complicated: the number of process steps and the number of process layers on the substrate increase more and more. Some of these layers also introduce asymmetries in the substrate grating mark and hence alignment errors.
Moreover, when the known on-axis alignment method is used, strict requirements must be imposed on the depth of the grating grooves of the substrate mark which is a phase grating.
It is an object of the present invention to provide an alignment system for a lithographic projection apparatus in which the influence of said effects on the alignment signal is reduced considerably and which is more accurate and reliable than known alignment systems. To this end, the system according to the invention is characterized in that the alignment mark is a diffractive mark and in that the alignment unit is adapted to separately detect a number of at least three sub-beams diffracted by the diffractive mark in different diffraction orders which are higher than 0, each sub-beam comprising an indication about the position of the substrate mark with respect to the reference.
Adapting the alignment unit such that it separately detects at least three subbeams with different diffraction orders does not mean that three or more sub-beams have to be detected simultaneously, but means that the alignment unit provides the possibility to separately detect all these sub-beams. In practice sub-beams can be detected or not and simultaneously or not.
A diffractive mark is a mark that splits a beam of electromagnetic radiation into a number of sub-beams of different diffraction orders. Such mark may be comprise a diffraction grating or by another diffraction element.
The invention is based on the recognition that better use can be made of the properties of a diffractive alignment mark if this mark is no longer used in combination with an on-axis alignment unit but in combination with an off-axis alignment unit. As described in U.S. Pat. No. 4,251,160, an image of the substrate grating having a period which is half the period of the substrate grating itself is obtained in an on-axis alignment unit in which a diffraction grating is used as a substrate alignment mark and in which only the first-order sub-beams of the substrate grating are used for imaging this mark on a corresponding grating alignment mark. Consequently, the alignment accuracy is twice as great as in the case where also the zero-order sub-beam and the higher-order sub-beams would jointly be used for this image. In the alignment unit described in U.S. Pat. No. 4,251,160, the first-order sub-beams are selected by an order diaphragm incorporated in the projection lens. Such a diaphragm considerably complicates the design of the projection lens which is already complicated anyway, and providing the projection lens with an order diaphragm which also passes higher:r orders, i.e. orders higher than 1, is not very well possible. By providing a diffractive alignment mark in an off-axis alignment unit, a great extent of freedom is created to select higher-order sub-beams of this alignment mark. The fact that, as a higher order is selected, the resolving power of the alignment unit is enhanced, may then be used to advantage.
The invention is also based on the recognition that the higher-order subbeams are determined by the edges of a grating mark rather than by its center and that, as compared with the center, these edges are less vulnerable to the CMP process or to other measures affecting the asymmetry of the grating. By using the higher-order sub-beams, not only the problem of asymmetric alignment gratings is eliminated, but the accuracy of the alignment unit is also enhanced.
It is to be noted that it is known from U.S. Pat. No. 4,828,392 to make use of a plurality of higher-order sub-beams from the substrate mark for aligning a substrate with an asymmetric alignment mark. However, the asymmetric mark is a mark whose grating grooves are asymmetrical with respect to the centerline of these grooves, and use is made of an order diaphragm which must be incorporated in the projection lens system and must be provided with a large number of apertures. The quality of the image of the mask pattern proper on the substrate is thereby undoubtedly affected.
U.S. Pat. No. 5,477,057 also describes an off-axis alignment unit for a scanning lithographic apparatus. A separate alignment sensor head for aligning a substrate mark with respect to an apparatus reference is arranged next to and against the projection lens system. The reason for using an off-axis alignment unit is the wish to use a wide-band alignment radiation which cannot adequatly be transmitted by the monochromatic projection lens system. The Patent does not describe a diffractive alignment mark or states the use of a plurality of higher-order sub-beams during alignment.
U.S. Pat. No. 4,870,452 describes an off-axis alignment unit for the substrate in which a plane-parallel plate is arranged between the projection lens system and the substrate. This plate is completely transparent to the projection beam but has a different reflection and transmission coefficient for the alignment beam. This plate must ensure that the alignment beam is incident at an acute angle to the substrate alignment mark and, after reflection, is directed at a given angle onto the detection system by this mark. The substrate mark may be, for example a grating mark and separate detectors may be provided for the sub-beams formed by this mark with different diffraction orders. However, only the 0.sup.th -order, the 1.sup.st -order and the 2.sup.nd -order sub-beams are used for the detection. It is not clear why beside the 1.sup.st -order, also the 0.sup.th -order and the 2.sup.nd -order sub-beams are used. In practice, it will be preferred not to provide a plane-parallel plate between the projection lens system and the substrate and it is doubtful whether this plate can separate the different orders to a sufficient extent.
A preferred embodiment of the projection apparatus according to the invention is further characterized in that said reference consists of a structure of a number of separate reference elements equal to the number of used diffraction orders and having the same shape as the substrate alignment mark, and in that a separate detector is associated with each of these elements for converting the sub-beam coming from the substrate mark and passed by the relevant diffractive reference element into an electric signal.
The apparatus may further be characterized in that the reference elements are gratings.
The alignment detection is then based on a grating-to-grating image which has proved to be reliable in the past.
To achieve that the sub-beams of the different diffraction orders can be detected in a well-separated manner without the alignment unit becoming too voluminous, the apparatus is preferably further characterized in that the radiation path between the substrate mark and the reference elements successively incorporates a first lens system, a structure of deflection elements arranged the paths of the sub-beams from the first lens system to give the separate sub-beams different directions, and a second lens system arranged behind said deflection elements for concentrating the sub-beams on the associated diffractive reference elements.
This embodiment is preferably further characterized in that the distance between the plane of the substrate mark and the first lens system is equal to the focal length of the first lens system, in that the distance between the second lens system and the plane of the reference elements is equal to the focal length of the second lens system and in that the distance between the first lens system and the second lens system is equal to the sum of the focal length of the first lens system and that of the second lens system.
The two lens systems together then constitute a telecentric lens system and the axial position of the structure of deflection elements is not critical anymore.
The apparatus is preferably further characterized in that the structure of deflection elements comprises a pair of deflection elements for each diffraction order to deflect the sub-beams of this diffraction order with opposed diffraction order signs such that the second lens system converges these sub-beams on one associated reference element.
Then, both the plus orders and the minus orders of the substrate mark are used for imaging this mark on the reference elements and optimal use is made of the available alignment radiation.
Several embodiments of the structure of deflection elements are possible. A first embodiment is characterized in that it comprises a number of discrete optical wedges, which number is equal to the number of sub-beams.
These wedges may be manufactured as separate elements and then fixed to a common transparent carrier plate. Preferably, the discrete wedges are manufactured in one process step by a replication technique, well-known in the optical art. A negative of the whole structure of wedges, present in a mould, is then printed in a layer of synthetic, for example, UV curable material provided on, for example, a plate of quartz.
Severe requirements are to be set to the mutual accuracy of, for example, the slope of the wedge surfaces of two wedges which are used for deflecting the +order and -order sub-beams of the same diffraction order. These requirements can be satisfied more easily in a second embodiment of the structure of deflection elements which is characterized in that it comprises a number of wedge-shaped transparent plates which are arranged one behind the other in the path of the sub-beams and have different wedge angles and a number of openings to pass radiation undeflected, the number of openings and their positions being such that with a combination of n plates 2.sup.n diffraction orders can be deflected in different directions in a binary way.
These wedges have the advantage that they can be manufactured relatively easily with the desired precision.
The substrate alignment mark may be a linear grating. When designing such a grating, the desired distribution of the radiation among the different diffraction orders can be taken into account.
To be able to align in two mutually perpendicular directions, the apparatus is further characterized in that the substrate mark comprises two grating portions, in which the direction of the grating strips of the first grating portion is perpendicular to that of the grating strips of the second grating portion in that the structure of deflection elements is a two-dimensional structure and in that the reference is a two-dimensional reference.
The apparatus is preferably further characterized in that the off-axis alignment unit comprises two radiation sources which supply beams of different wavelengths, and a beam splitter for combining the two beams on their path to the substrate mark and for splitting the beams reflected by this mark, and in that a separate structure of deflection elements and reference elements is present for each of these beams.
It is then not necessary to impose strict requirements on the groove depth of the substrate mark.
A practical embodiment of the apparatus is further characterized in that a second off-axis alignment unit is present, and in that the first-mentioned alignment unit and the second alignment unit are arranged diametrically with respect to the projection system.
The off-axis alignment system can also be used for aligning a further alignment mark on the substrate holder with respect to the reference. Then the alignment of the substrate mark with respect to the substrate holder mark can be determined.
To be able to realize the ultimate alignment of the mask pattern with respect to the substrate, the apparatus is further characterized in that the alignment system also comprises an on-axis alignment unit for aligning the substrate holder with respect to the mask pattern.
The on-axis alignment unit may be of the type comprising a radiation source which emits a beam having a wavelength which is different from that of the projection beam, but may be alternatively constituted by an image sensor operating with projection radiation.
The invention can also be used in lithographic apparatus which comprises a projection station for projecting a mask pattern onto a first substrate and a measuring station for measuring the position of a second substrate. An embodiment of such an apparatus, having two substrate stages which commute between the measuring station and the projection station is shown in U.S. Pat. No. 4,861,162. Such apparatus has the advantage that its throughput, i.e. the number of wafers that can be processed in one hour, is considerably larger than that of a comparable apparatus having only a projection station and no measuring station.
A dual station lithographic projection apparatus wherein the invention is implemented is characterized in that the measuring station comprises an off-axis alignment unit as described above.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.